Former Vice President Joe Biden in New Hampshire on Tuesday said House Democrats probing allegations of obstruction by President Donald Trump are getting stonewalled by the administration and impeachment proceedings against the president could follow “very quickly,” Fox News reported.

Biden made the comment during an interview following a campaign stop.

He added there may be “no alternative” but to move forward with impeachment proceedings, after the White House on Tuesday instructed two former aides, Hope Hicks and Annie Donaldson, to defy congressional subpoenas related to allegations the president obstructed justice. Hicks has said she will comply with the subpoena.

Biden’s comments are the strongest yet from the former vice president on the controversial impeachment issue. Other Democratic candidates for the 2020 presidential primary already have come out in support of impeachment, including Sens. Bernie Sanders, I-VT, Elizabeth Warren, D-Mass., Kamala Harries, D-Calif., and Cory Booker, D-N.J. South Carolina Rep. Jim Clyburn, the third ranking Democrat in the House, told CNN on Sunday he believes an impeachment proceeding ultimately will be launched in the House.

House Judiciary Chair Jerrold Nadler, D-N.Y., whose committee would lead an impeachment proceeding, said during a radio interview last week “there certainly is” reason to launch an impeachment inquiry but that it was important that the American people were supportive before the process began.

Fifty-nine House Democrats have publicly signed on in favor of pursuing an impeachment inquiry, 64 have said no or not now or remain undecided, and 112 have taken no position, according to the New York Times. While there are a number of former Republican government officials who have come out in favor of impeachment, such as former White House Counsel John Dean, who figured prominently in the Watergate proceeding, only one sitting congressman, Rep. Justin Amash, R-Mich., has been willing to break from the party line that there is no reason for the inquiry.

This level of “tribalism,” as a Washington Post analysis piece last week referred to the Republican's steadfast position against impeachment, has not always been so. When Congress faced an impeachment proceeding 20 years ago involving Democrat Bill Clinton, 30 Democrats voted to open the proceeding, and 30 Republicans voted not to impeach the president.

Biden said in New Hampshire Democrats have a “responsibility to move if in fact they are unable to get the data that is totally within their power to be able to subpoena before the Congress to make a judgment.” He added, if stonewalled, the only “constitutional avenue for them is impeachment.”

Special Prosecutor Robert Mueller, a lifelong Republican, in a televised statement last week said his report did not exonerate the president of obstruction of justice. That statement has put increased pressure on Speaker of the House Nancy Pelosi, D-Calif., and she has remained non-committal on the issue because “we want to do what is right and what gets results.”