Tomorrow’s primaries in New York and six other states — the last of the 2010 cycle, save for Saturday’s primary in Hawaii and a runoff in Louisiana on Oct. 2 — originally looked as though they’d have little impact on the electoral landscape. Although some of the primaries were nominally competitive – like the Republican Senate race in Maryland, where a suite of 11 candidates will be on the ballot — they were typically not so in places where the nominee stood a strong chance of winning the general election.
Two races, however, have changed that equation. And depending on how they are resolved, Republican chances of taking over the Senate could be enhanced or significantly diminished.
The first race in Delaware, where Christine O’Donnell, a political activist and commentator, is running against Michael N. Castle, who has held elected office in Delaware for 30 years as its governor, lieutenant governor and lone United States representative. The contest originally appeared to be a mismatch. Republican strategists were thrilled to have a well-known and manifestly electable candidate like Mr. Castle, who announced his interest in running for the Senate last October. Ms. O’Donnell, meanwhile, was far from a fresh face, having been the Republican nominee for Senate in 2008, when she was badly defeated by Joseph R. Biden Jr., now, of course, the vice president. Ms. O’Donnell was also on the ballot for the Republican Senate primary in 2006, finishing in third place with barely more than 2,500 votes.
But then a strange thing happened. Joe Miller, who had the backing of Sarah Palin and the Tea Party Express, upended the incumbent Lisa Murkowski in the Republican Senate primary in Alaska. Although the outcome arguably should not have been such a surprise – a dearth of polling concealed whatever momentum Mr. Miller might have been gaining – it emboldened Tea Party activists and some other conservatives, who were reminded that in this topsy-turvy electoral cycle, few incumbents and establishment politicians are safe. Mr. Castle — a moderate who is unambiguously a member of the establishment – was next on their target list. And so Ms. O’Donnell, who already had the support of the Tea Party, last week received endorsements from Republican thought-leaders like Ms. Palin, the National Rifle Association and Senator James DeMint of South Carolina.
In contrast to Alaska, however, where Mr. Miller is the favorite to be elected unless Ms. Murkowski finds her way onto the ballot as a Libertarian or write-in candidate, Delaware is a blue state, and the electoral prospects of Mr. Castle and Ms. O’Donnell there are wildly divergent. Whereas Mr. Castle is nearly a 95 percent favorite against the Democratic nominee, Chris Coons, according to last week’s FiveThirtyEight forecasting model, Ms. O’Donnell would have just a 17 percent chance of winning a race against Mr. Coons.
A parallel version of this chain of events exists in New Hampshire, where Kelly Ayotte, the state’s attorney general, was expected to be the Republican standard-bearer in the Senate race there. Although Ms. Ayotte’s matchup against the Democratic nominee, Representative Paul W. Hodes, remains competitive, she has led in every poll of the race for more than a year, and is pegged as better than an 85 percent favorite by the FiveThirtyEight model. Another Republican candidate, however, the attorney Ovide Lamontagne, has trailed in most polls against Mr. Hodes and would be a modest underdog to defeat him.

But like Ms. O’Donnell, Mr. Lamontagne has won the backing of Mr. DeMint and several Tea Party groups (although not that of Ms. Palin, who endorsed Ms. Ayotte in July). And also like Ms. O’Donnell, he appears to have some late momentum, with one poll putting him just 7 points behind — an uncomfortably small margin in a late-developing primary. Primary polls thus far in 2010 have missed the final margin between the candidates by an average of about 10 points, so Mr. Lamontagne is well within striking distance.
Ms. O’Donnell, for her part, already leads in at least one poll of Delaware. Although her lead in the survey, from Public Policy Polling, was within the poll’s margin of error, it is probably right to consider her a very slight favorite to defeat Mr. Castle based on the sharp upward trajectory in her numbers — a phenomenon that can sometimes be dispositive in primaries, where voters may jump on the bandwagon of a once-obscure candidate who suddenly appears to be viable.
The primaries in Delaware and New Hampshire have implications far beyond their borders. The forecast model that we ran last week gave Republicans a 26 percent chance of taking over the Senate — and enough states are tossups that they would be well within reach of doing so if the elections were held today. But this forecast was based on a weighted average likelihood of various candidates winning their primaries — for example, we had estimated that Ms. O’Donnell had a 25 percent chance of prevailing in Delaware, and Mr. Lamontange a 30 percent chance of doing so in New Hampshire — leaving Mr. Castle and Ms. Ayotte as the favorites.
If Ms. O’Donnell and Mr. Lamontange were both to win their primaries, however, the Republican chances of a Senate takeover would fall to just 16 percent, according to the model. Conversely, if Mr. Castle and Ms. Ayotte were to win, Republicans chances would rise to 30 percent. Thus, Republican prospects of claiming the Senate could be nearly halved if both the insurgent candidates were to prevail. They would still have a chance of doing so — but it might require them to expand the playing field further, like to West Virginia or Connecticut, where the Democratic candidates are clear favorites but not prohibitive ones — or to perform a quick makeover on a candidate like Ms. O’Donnell, who is unlikely to make a good first impression with moderate and independent voters.
Nevertheless, it should be remembered that were it not for the enthusiasm of the Tea Party — and other conservative voices outside the Republican establishment — the party might not be in the intriguing position that it finds itself in heading into November. Although Republicans enjoy many advantages in this cycle, perceptions of the party itself is not one of them. Nor are most figures who are identified with the party establishment popular. Instead, it has been the party’s occasional success at portraying itself as consisting of ordinary, aggrieved, disempowered outsiders – a new type of ‘silent majority’ — that have freshened its message for some voters.
Meanwhile, some Tea Party-backed Senate candidates — like Marco Rubio in Florida and Rand Paul in Kentucky — have been improving their position in polls and are now favorites in their races. But while Mr. Rubio has charisma, and Mr. Paul has the advantage of running in a state in which President Obama is very unpopular, neither of those advantages would hold for someone like Ms. O’Donnell, whose nomination would represent a clear setback for Republican chances of taking over the Senate. But few voters in either party seem in much mood for measuring their risk.