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Our new Senate forecast gives the Republicans a 77 percent chance of a Senate majority. That's down slightly from our previous estimate, which pegged the GOP's chances of a takeover at 82 percent. So, what's changed?

The new projection reflects the conclusion of several primary elections -- North Carolina, Nebraska, and West Virginia -- and, more importantly, the inclusion of candidate fundraising through the first quarter of 2014 and, for some races, through the end of April. (The relevant Federal Election Commission data are here.)

Our earliest forecast showed that Republicans were already heavily favored due to the national landscape and the partisan complexion of the states holding Senate elections this year. We then showed that incorporating a measure of the "quality" of the candidates -- prior experience in elective office -- made things even more favorable to Republicans. Chris Cillizza and I discussed that forecast here. As we would expect, Republicans are recruiting and nominating relatively experienced and therefore more electable candidates.

Now, with fundraising in the model, the results are marginally more favorable to Democrats, but not by much. This means that Democrats are mustering some advantages in fundraising, but not particularly large ones.

Because so many states have not had primaries, we measure fundraising by summing up all the fundraising by Senate candidates in each party. We expect that many donors to primary candidates who lose will likely end up supporting the candidate who wins, as will donors who didn't contribute in the primary. For example, between January 2013 and the middle of April, North Carolina Sen. Kay Hagan (D) raised about $11 million. Her opponent, Thom Tillis, raised $3.2 million. Is Hagan likely to out-raise Tillis nearly 4-1 until November? Most likely not. In fact, our analysis of past elections suggests that at this stage party fundraising is a better predictor than fundraising by the primary candidates that go on to compete in the general election. (My collaborator and fellow political scientist Eric McGhee will say more about this in a post on the Monkey Cage Blog next week.)

By including fundraising, a host of states that were already looking good for Democrats now look even better. For example, in Oregon, Delaware, New Jersey, Minnesota, and New Mexico, the chances of a Democratic victory are 10 points greater than our last forecast. We estimate that in all these states the Democrats chances of winning are about 90 percent or better.

At the same time, our forecast in the key competitive races is less optimistic for Democrats. The model still sees Democrats as slight underdogs in Arkansas, Alaska, and Michigan. But "slight" is the operative word.

In Iowa, including fundraising in the model does improve the chances of Democrat Bruce Braley, but the model still sees Iowa as a 50-50 toss-up. The resolution of the primary there will likely increase Braley's chances, since the Republican nominee will have less prior political experience than the model currently assumes. (We use historical averages from open-seat races to generate an estimated level of candidate experience in races where the primary hasn't been held. All of the GOP Senate candidates in Iowa have less prior political experience than the historical average.) However, the model is still likely to see the race as fairly competitive.

Only in North Carolina is the model relatively favorable to the Democrats, giving Hagan a very solid shot at defeating Tillis. This largely reflects historical precedent: it is very rare for a state legislator to defeat an incumbent Senator. Of course, that doesn't mean it won't happen. But Tillis may have his own challenges too.

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Republicans have a 77% chance of taking the Senate