Since John McCain wrapped up the Republican nomination, it's become clear that a small but significant number of pro-McCain Republicans have begun participating in the Democratic primary and are voting for Hillary Clinton.

It's a trend that will have an impact on the results tonight in Indiana, though at this point we don't know how big. It's also not clear whether these Republicans are engaged in Operation Chaos-style "strategic voting" or if they are expressing their preference for second place.

Comparing the numbers from before McCain effectively secured the GOP nomination to after, it appears that since March, up to 3% of all voters in open Democratic primaries are Republicans supporting Clinton with no intention of actually voting for her in the general.

Based on numbers I compiled this afternoon, before the McCain wrapped up the nomination, just under 2% of voters in Democratic open primaries were Republicans for Clinton. Since then, the number has jumped to 4.6%. Meanwhile, Obama's number has remained steady at 4.5%, indicating his Republican supporters are mostly genuine Obama backers. (Pro-Obama Republicans who voted in the early primaries had to give up voting in the Republican contest.)

After the Mississippi primary, Mark Blumenthal was able to get his hands on the cross-tabs for the exit poll. Here's what I wrote about those numbers back in March:

Blumenthal, who got his hands on exit poll cross-tabs, found that 85% of Republiclintons had a favorable view of John McCain. 58% had a strongly favorable view.

72% of Republiclintons said Clinton was not "honest and trustworthy." 62% said she does not inspire them, 56% said she has not offered clear and detailed plans, and even 41% of them would be dissatisfied if she were to win the Democratic nomination (!).

These numbers strongly indicate that Clinton's Republican supporters do not plan to vote for her. At the same time, the numbers also showed that Clinton was clearly the second-choice of most of these Republicans (behind McCain), suggesting that their preference for Clinton over Obama was genuine, even though they plan to vote for McCain.

I restricted my analysis to states where there were open primaries and where the exit polls had a sufficient sampling of Republicans to show vote preferences. In all, eight states met this criteria: Alabama, Mississippi, Missouri, Ohio, South Carolina, Texas, Virginia, and Wisconsin. (Technically, Ohio is not open, but voters can change registration on the day of the vote, and the laws are very loosely enforced.)

Here's the weighted average of how the impact of Republicans from those states (remember, numbers may not add up due to rounding):

• Republicans as % of all voters: 8.2%
  
• Rep. for Clinton as % of all voters: 3.4%
  
• Rep. for Obama as % of all voters: 4.5%

Now, what about change over time?

First, let's take a look at the same numbers for the five states who voted while there was still a contested GOP primary (AL, MO, SC, VA, WI):

• Republicans as % of all voters: 6.7%
  
• Rep. for Clinton as % of all voters: 1.8%
  
• Rep. for Obama as % of all voters: 4.5%

Now, let's look at the numbers for the three states that voted after McCain had wrapped up the nomination and Limbaugh began Operation Chaos:

• Republicans as % of all voters: 9.2% (+2.5%)
  
• Rep. for Clinton as % of all voters: 4.6% (+2.8%)
  
• Rep. for Obama as % of all voters: 4.5% (unch.)
  

This data is pretty compelling evidence that once the Republican primary ended, GOP voters who would have voted in a contested Republican primary, crossed over into the Democratic primary to support the Clinton campaign. Obama did not receive a similar boost.

It seems almost certain that these new Republicans for Clinton would vote for John McCain in the fall. (She would likely retain the support of most Republicans who supported her prior to March.)

It's unclear whether these voters are picking Clinton as a strategic move or because they legitimately prefer her as a second choice to McCain instead of Obama, perhaps motivated by Republican groups like the National Right to Life Committee, which robocalled against Obama.

Regardless, the strong likelihood that they will end up voting for McCain is problematic, because pro-McCain Republicans should not have an impact on the Democratic primary.

Fortunately, this won't be an issue in North Carolina tonight, but it will be significant issue in Indiana.

I've updated this entry a bit since I originally posted it, adding in the numbers from Blumenthal.