President Trump claimed without evidence Tuesday that he is “leading in the polls” as he sought to push back against rising calls for impeachment from Democrats amid a burgeoning controversy over his July call with the leader of Ukraine.

“I think it’s ridiculous. It’s a witch hunt,” Trump said of the impeachment calls as he arrived at the United Nations in New York. “I’m leading in the polls. They have no idea how they stop me. The only way they can try is through impeachment. This has never happened to a president before.”

Recent national polling has shown Trump trailing former vice president Joe Biden and several other Democrats, while polling in several battleground states has not presented a much brighter picture for the president.

A Trump campaign spokesman did not immediately respond to a question about the polls to which Trump was referring.

The list of Democrats calling for Trump’s impeachment has been bolstered in recent days by revelations that Trump asked Ukrainian President Volodymyr Zelensky to investigate Biden and his son in a phone call that took place shortly after Trump had temporarily suspended military aid to the country.

Democrats have accused Trump of trying to pressure a foreign power to hurt Biden, now a leading Democratic presidential contender. Trump has acknowledged he held back the aid, but denies doing anything improper. On Tuesday, he said he was concerned the United States was contributing more to Ukraine than European countries. He has previously said he was worried about corruption in Ukraine.

In surveys of registered voters since the start of August, Trump has trailed Biden by 14 percentage points in a Fox News poll, by 15 points in a Washington Post-ABC poll, by 12 points in an Investor’s Business Daily/TIPP poll and by 16 points in a Quinnipiac University poll.

When matched against four other Democratic candidates — Sen. Warren Elizabeth Warren (Mass.), Sen Bernie Sanders (I-Vt.), Sen. Kamala D. Harris (Calif.) and South Bend, Ind., Mayor Pete Buttigieg — Trump did not have an advantage against any of them. At best he trailed within the range of sampling error.

Recent polling has also shown Trump without a clear lead against top Democrats in several of the mostly closely contested states in 2016.

In Michigan, for instance, an EPIC-MRA poll in August asked whether registered voters would support Trump against four different Democratic candidates.

Trump trailed Biden by 10 points, a statistically significant edge. Warren and Sanders held slight edges over Trump of 4 points and 5 points, respectively, while Harris had a 3-point lead, which was within the margin of error.

In Wisconsin, a Marquette Law School poll in August found Trump tied or behind Biden, Sanders, Warren and Harris among registered voters. Biden led Trump by 9 points; Sanders had a slight 4-point edge, while Warren and Harris were tied with Trump.