Rep. Joaquin Castro (D-Texas) is calling for President Trump to back up his claim that former President Barack Obama Barack Hussein ObamaGOP senator blocks Schumer resolution aimed at Biden probe as tensions run high D-Day for Trump: September 29 Obama says making a voting plan is part of 'how to quarantine successfully' MORE wiretapped Trump Tower before the election or apologize to his predecessor.

"President Trump should back up his wire tapping claim immediately or apologize to President Obama and the nation," Castro tweeted Monday.

"I've seen no evidence."

President Trump should back up his wire tapping claim immediately or apologize to President Obama and the nation. I've seen no evidence. — Joaquin Castro (@JoaquinCastrotx) March 6, 2017

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Trump, citing no evidence, claimed on Saturday morning that Obama had his phones tapped in Trump Tower before his presidential victory.

Trump questioned whether it was legal for a sitting president to be "'wire tapping' a race for president prior to an election."

"How low has President Obama gone to tapp my phones during the very sacred election process," the president tweeted. "This is Nixon/Watergate. Bad (or sick) guy!"

White House press secretary Sean Spicer on Sunday called the reports about "potentially politically motivated investigations" before the 2016 presidential election "very troubling."