Britney Spears' ex-Husband Jason Alexander Part of Capitol Protest

Britney Spears' ex-husband -- the one who isn't Kevin Federline -- had a front-row view to the attempted coup at the Capitol ... because he was part of the pro-Trump protest.

Jason Allen Alexander -- whom you might remember for being married to Britney for 55 hours after their wild, 2004 Sin City wedding -- trekked to D.C. for Wednesday's show of support for President Trump, which turned into an attempted coup.

Jason looked proud to be there ... snapping selfies in his Trump 45 beanie.

What's unclear ... if Jason actually was part of the riotous mob that breached the Capitol. If he was, it's likely the feds are going after him.

Seems Jason's big on attending demonstrations ... when we last saw him back in August, he had joined the #FreeBritney movement ... hanging out in the crowd gathered outside the L.A. courthouse where a hearing was taking place on her conservatorship.