LinkedIn's (LNKD) Reid Hoffman is joining the chorus of those calling for Donald Trump to release his tax returns -- and he's putting $5 million behind his voice. The move echoes one made by Trump himself in 2012.

The cofounder and executive chairman of the professional social network Monday published a Medium post challenging Trump to release his tax returns. Trump has so far argued that he should not do so because he is under audit. Hoffman has tossed his support behind Pete Kiernan, a 26-year-old Marine Corps veteran who on crowdfunding platform Crowdpac.com has launched a campaign to raise at least $25,000, which he says he will donate to non-profit organizations that assist veterans if Trump releases his tax return by the final debate, October 19.

Hoffman, who is an investor in Crowdpac, pledged Monday that if Kiernan's campaign meets or exceeds its target, he will match the total amount he reaches with a contribution of five-times that amount, up to $5 million.

"By releasing his tax returns by the third and final debate (October 19), Trump can help voters make more informed decisions on Election Day and support many worthy veterans' organizations at the same time," Hoffman wrote.

The money raised, if Trump releases his returns, will go to organizations like Team Rubicon, the Special Ops Warrior Foundation and the Yellow Ribbon Fund.

In extending the offer, Hoffman is tearing a page out of Trump's own playbook. The real estate magnate in 2012 offered $5 million to President Barack Obama to reveal his transcripts and passport application.

He is also hitting at a spot that is especially sensitive for Trump: military veterans. The Republican presidential nominee has cast himself as a champion of veterans on the campaign trail.

Last year, he skipped a primary debate to host a televised fundraiser for veterans. Months later, reporters were hard pressed to find where the $6 million he claimed to have raised had gone, and Trump gave the $1 million he pledged only when pushed. He caused a stir by attacking the parents of fallen U.S. army captain Humayun Khan following the Democratic National Convention.

The purpose of Kiernan's push, and Hoffman's subsequent backing, is, of course, to pressure Trump, who often touts his support his love for the military and veterans, to release his returns in order to benefit a cause about which he claims to care deeply. But Trump has remained vehement that he has no plans to release any documentation -- even what's no longer under audit -- until an IRS audit on all of his returns is complete.

To be sure, Hoffman is not exactly a neutral party in all of this. He was one of numerous business leaders to back Hillary Clinton's presidential bid in June, in a statement calling the former secretary of state "the candidate who best ensures the future of American prosperity." He is a major Democratic Party supporter and donor, giving $1 million to Priorities USA Action, the super PAC now backing Clinton that previously supported President Obama, in 2012. He has contributed to the Clinton campaign this election cycle, and who knows, he could even be up for a spot in her presidential cabinet.