Activists around US hold rallies supporting Merrick Garland's Supreme Court nomination

Liberal groups allied with the White House staged rallies around the country on Monday to pressure Republican senators including the Judiciary Committee's chairman not to block President Barack Obama's Supreme Court nominee Merrick Garland.

At the same time, a conservative group, the Judicial Crisis Network, said it launched a $2 million, three-week television, radio and digital advertising campaign in Colorado, Indiana, Iowa, New Hampshire, North Dakota and West Virginia opposing Garland's confirmation.

The group is backing the stance of Senate Republicans including Majority Leader Mitch McConnell not to hold confirmation hearings or a vote on Garland's nomination.

In Iowa's capital, demonstrators targeted Judiciary Committee Chairman Chuck Grassley, whose panel handles confirmation hearings for Supreme Court nominees.

More than 50 protesters gathered on the sidewalk outside a federal building where Grassley has an office, chanting "do your job" as a plane pulling a banner carrying the same message flew overhead.

Liberal groups including MoveOn.org organized rallies targeting Republicans in their home states during the Senate's two-week spring recess in states including Ohio, Pennsylvania, Texas and Wisconsin. The rallies generally drew crowds in the dozens, organizers said.

"I have called Grassley's office every week for the last two or three weeks and it doesn't seem like he's getting the message, so I need to find another way to do this," said Des Moines demonstrator Tracey Aubrecht, a 44-year-old homemaker from North Liberty, Iowa.

State Senator Rob Hogg, one of the four Democrats running in a primary for a chance to challenge Grassley in the Nov. 8 election, attended the rally.

The nomination requires confirmation by the Republican-led Senate. Republicans have said they want the next president to make the selection, hoping their party wins the presidential election.

Garland, a centrist appellate judge and former prosecutor, was picked last week to replace Justice Antonin Scalia, who died Feb. 13. His appointment could tip the court leftward for the first time in decades.

The White House said Vice President Joe Biden will deliver remarks on Garland's nomination at Georgetown Law School on Thursday. It said Garland also will continue meeting senators individually, speaking with Democrats Charles Schumer of New York and Bob Casey of Pennsylvania on Tuesday and Amy Klobuchar of Minnesota on Wednesday.

Both sides have promised intensive campaigns on the ground and over the airwaves that will eclipse other battles over Supreme Court nominees in recent decades.

Garland remains relatively unknown among Americans. Roughly one out of every five people say they are "somewhat" or "very" familiar with him, a Reuters/Ipsos poll conducted from Thursday to Monday found. Among those who have heard of Garland, half expressed a favorable opinion, including 60 percent of Democrats and 43 percent of Republicans.

(Reporting by Kay Henderson; Additional reporting by Timothy Gardner and Chris Kahn; Editing by Will Dunham)