US President Joe Biden's administration voiced strong support on Tuesday for its nominee to become the Pentagon's top policy adviser, who is a prime target of Republicans aiming to torpedo several Biden picks over their social media activity.

Colin Kahl, who served as national security adviser to Democrat Biden when he was vice president, is his first Pentagon nominee to come under serious pressure after Republicans portrayed him as hyper-partisan.

That is a characterization that Kahl's supporters firmly reject.

In an apparent sign of the administration's concerns about the risks to Kahl's nomination in the evenly divided Senate, Defense Secretary Lloyd Austin spoke on Tuesday with Senator Joe Manchin, a Democratic member of the Senate Armed Services Committee who told Fox News he was still undecided on Kahl. A committee vote on Kahl could come as early as this week.

The Biden administration would need to ensure Kahl at least has half the votes on the committee in order to advance his nomination to a full Senate vote, congressional sources said.

"This is a critically important position ... And Dr. Kahl is eminently qualified to fill it," Pentagon spokesman John Kirby told a news briefing.

"The secretary fully supports his nomination and wants him installed as soon as possible."

Republican Senator Tom Cotton, during Kahl's March 4 confirmation hearing, lambasted Kahl for warning on Twitter of the risk of conflict with Iran after the Trump administration killed top Iranian General Qassem Soleimani last year.

He said Kahl had spent "the last four years warning about impending wars that never happened."

Cotton also pointed to a tweet by Kahl describing Republicans as "the party of ethnic cleansing" after Trump abruptly ordered a Syria withdrawal in 2019, in a move that many said put at risk ethnically Kurdish allies.

Kahl apologized in the hearing and said the last several years had been "pretty polarizing on social media."

"I'm sure there are times that I got swept up in that," he said.

Michele Flournoy, a former senior Pentagon official who enjoys strong bipartisan support on the Senate Armed Services Committee, stressed that Kahl had proven himself as the top Pentagon policy official for the Middle East from 2009-11.

"Colin Kahl has a clear track record of effective, nonpartisan leadership in the Department of Defense," Flournoy said. "His behavior and actions in public service should be given more weight than a selective reading of his tweets."

Kahl is one of several Biden nominees to be scrutinized over his Twitter feed. Neera Tanden withdrew from consideration last week as Biden's nominee for Office of Management and Budget director after criticism for previous social media posts.

Vanita Gupta, Biden's nominee for the Justice Department's No. 3 post, was accused by a top Republican at her confirmation hearing on Tuesday of excessive partisanship over her Twitter posts criticizing Republicans.