Why do Democrats many times pretend to be more conservative during campaigns than they actually are?

You don't see Republicans pretending to be liberal during their campaigns, but you see Democrats pretending to be conservative.

1) “People just can’t know that.” MO Sen. McCaskill Hides Agenda Including “semi-automatic rifle ban” from Moderate Voters, Staffers Reveal in Undercover Video it “could hurt her ability to get elected.”

2) "Rep. Beto O’Rourke often touts his mother in speeches and interviews as a lifelong Republican, who now supports his effort to unseat Republican Sen. Ted Cruz of Texas. Not really. . . . But according to CNN, Mrs. O’Rourke cast a ballot on the Democratic side of 15 of the last 17 primaries in which she voted, including Democratic primaries that included presidential contests as far back as 2000. . . . According to CNN, her donations to Democrats included $250 to Mr. Obama’s campaign, $1,200 to Ronald Coleman, a Democratic congressman from Texas, from 1983 to 1990."

3) Regarding the votes on confirming Justice Kavanaugh, take the two votes: the first vote on the rule and then the second vote on the actual confirmation. On the cloture vote, Senator Joe Manchin waited until after it was clear that there were 50 votes for Kavanaugh. As to the vote for confirmation, Manchin waited until right after Senator Collins announced at 3 PM that said she was voting “yes” before he declared his vote so again it was clear that his vote didn’t matter then.

4) Pretending to support the NRA, but only when they know that their vote won't matter. Then Senator Mary Pryor in 2009:

Mark Pryor knows all about that. The Democratic senator from pro-gun Arkansas was nowhere to be seen on the Senate floor during Wednesday's showdown over a proposal, championed by the National Rifle Association, that would have gutted state gun-control laws across the nation. 

After a morning of angry speeches, a vote was called at high noon. Toward the end of the vote, Pryor entered the chamber through the back door, took a few steps inside, flashed a thumbs-down to the clerk, and retreated as fast and furtively as somebody dodging gunfire 

Several minutes later, the Democrats had racked up more than enough votes to block the proposal. "Are there any senators in the chamber wishing to vote or wishing to change their vote?" the presiding officer inquired. 

Pryor burst back in, this time through a side door. "Mr. President!" he called out. "Mr. President!" He stopped in the well to consult with Sen. Chuck Schumer (D-N.Y.), a gun-control advocate who was keeping the whip sheet. Schumer gave Pryor a nod, and the Arkansan -- reassured that his vote was not needed to defeat the proposal -- changed his vote to an "aye." . . .