Ah, the wonders of plurality-based voting. New York's 20th congressional district is having a special election, after their congressperson, Kirsten Gillibrand, was appointed to Hillary Clinton's senate seat (after Clinton became Secretary of State). It's a slightly-Republican-leaning district, but Eric Sundwall was running as a candidate for the Libertarian party. The Libertarians, of course, when compared to the two major parties, are most-similar to the Republicans, and so, thanks to plurality voting's susceptibility to spoilers, would likely have siphoned more votes from them than from the Democrats.
The Republicans in the state apparently felt that Sudwall's spoiler-effect might have been strong enough to sink the candidate's, Jim Tedisco, chances of wining, and so launched a coordinated effort to challenge Sundwall's petition signatures, and force him off the ballot. They were successfull. And so, Sundwall has endorsed the Democratic candidate, Scott Murphy. Tedisco's campaign is already considered to be much more negative than Murphy's, and this can't be helping.
Sundwall was only showing about 2% support in the election, with approximately equal support from Democrats and Republicans, so you have to wonder how much the Republicans thought this would help them. But in a close enough plurality election, a spoiler only needs a couple of votes to change the outcome. Under instant runoff, a candidate needs at least 25% support to spoil an election, and under score voting, spoilers don't exist.