US Democratic presidential nominee Hillary Clinton at Futuramic Tool & Engineering in Warren, Michigan, on Thursday. Reuters/Chris Keane WASHINGTON — US Democratic presidential nominee Hillary Clinton paid an effective federal tax rate of 34.2% last year and a combined federal, state, and local effective tax rate of 43.2%, her campaign said on Friday.

Clinton's running mate, Sen. Tim Kaine of Virginia, along with his wife, Anne Holton, paid a federal effective tax rate of 20.3% in 2015, the campaign said.

"Hillary Clinton and Tim Kaine continue to set the standard for financial transparency," Clinton campaign aide Jennifer Palmieri said in a statement. "In stark contrast, Donald Trump is hiding behind fake excuses and backtracking on his previous promises to release his tax returns."

It is customary for US presidential candidates to make their tax returns public, though they are not required by law to do so.

Clinton's tax returns have been made public, in some form, every year since 1977.

Trump and his lawyers have cited an audit by the Internal Revenue Service as a reason he has refused to release his returns. The Republican nominee has also said his taxes are no one's business and they reveal little.

The IRS has said Trump can release his tax returns even while under audit.

On Thursday, Trump's special counsel, Michael Cohen, told CNN he would not allow Trump to release them until the audits are complete.

Critics, including 2012 Republican presidential candidate Mitt Romney and other fellow Republicans, have said Trump's refusal raises questions about his net worth, his charitable contributions, his business dealings, and various other ties, including with Russia.

Clinton has pounced on the issue, releasing an online video on Friday highlighting high-profile Republicans urging Trump to release his taxes. On Thursday she raised the issue during an economic speech in Michigan.

"He refuses to do what every other presidential candidate in decades has done and release his tax returns," she told the crowd.

Politico has reported that Trump paid zero to very little taxes for two years in the 1990s, and a New York Times business columnist on Friday quoted numerous tax lawyers and accountants saying that could still be the case.

Clinton's campaign has released tax returns going back to 2007. Clinton, who now lives in Chappaqua, New York, paid an average effective federal tax rate of about 32% from 2007 to 2014 and an effective combined tax rate of approximately 40.5%.