Donald J. Trump is the presidential nominee of the Republican party. But that does not absolve every Republican office holder, donor, and activist from the responsibility of satisfying himself that it is right to support that nominee for president. There are, in my judgment, many reasons to doubt this is the case. But one reason in particular hasn't received sufficient consideration: The fact that Trump and his top campaign aide have many troubling connections with Vladimir Putin's regime.

Honest and patriotic Republicans who support Trump, or are tempted to do so, should review some of the publicly available evidence. Trump's business seems to be heavily dependent on Russian investment. His top campaign advisor, Paul Manafort, was the advisor to the Putin-backed stooge Viktor Yanukovich, and has deep ties to the Putin apparat. One of Trump's national security advisors, retired Lt. General Michael Flynn, was paid to give a speech at a Russian propaganda celebration and was seated next to Putin. Trump's Russia advisor Carter Page, who does much of his business with Russian companies, has argued, among other things, that "a few officials in Washington" annexed Ukraine and that the "so-called annexation" of Crimea by Russia was a rational response to this injustice.

Furthermore, practically the only change Trump's campaign made to the GOP platform was to weaken language supporting Ukraine. Wikileaks, which appears to have connections to Putin's espionage apparatus, has released emails that are damaging to Hillary Clinton immediately before the Democratic convention. Trump heartily approves of this interference by a foreign power in an American election. They apparently intend to do the same with emails hacked from Clinton Foundation servers. Finally, Trump, to the cheers of the Kremlin establishment, has said he will not uphold our NATO commitments.

These indications provide sufficient grounds for Trump's links to Putin to be further investigated. Politicians who are currently supporting Trump should withdraw their unconditional support "until," as Mr. Trump might say, "we can figure out what's going on." One thing that would help us figure out what's going on is if Trump—and Manafort—released their tax returns. They of course haven't done so. Indeed, it's striking that neither seems even to have put his current business interests in any kind of blind trust. So we don't know how direct and close a financial relationship Trump and Manafort have with the Putin regime.

If Trump and Manafort don't act to allay these concerns by releasing their tax returns (or in other ways), wouldn't it be advisable for a Republican member of Congress to lead an urgent investigation into whether Putin is interfering in the current American election? Trump and Manafort may be Putin's chumps. Will other Republicans sit by as the whole Republican party becomes Putin's party?