HERDING CATS…. It all seemed so encouraging. President Obama delivered a national address before both houses of Congress, outlining an ambitious and fundamental change to the way the government operates. Americans loved it. The White House then unveiled a budget blueprint that not only mirrors the campaign agenda endorsed by the electorate last year, but also takes the country in precisely the right direction.

The president is riding high; he enjoys a national mandate; his party controls both the House and Senate; Republicans can’t filibuster budget reconciliations; and the public finally trusts the Democrats to do what’s right on the economy. There was ample reason for optimism.

And yet, the old Will Rogers adage about Democrats rings true for a reason.

Moderate and conservative Democrats in the Senate are starting to choke over the massive spending and tax increases in President Barack Obama’s budget plans and have begun plotting to increase their influence over the agenda of a president who is turning out to be much more liberal than they are.

A group of 14 Senate Democrats and one independent huddled behind closed doors on Tuesday, discussing how centrists in that chamber can assert more leverage on the major policy debates that will dominate this Congress.

Afterward, some in attendance made plain that they are getting jitters over the cost and expansive reach of Obama’s $3.6 trillion budget proposal.

Asked when he’d reach his breaking point, Nebraska Sen. Ben Nelson, one of the most conservative Democrats in the Senate, said: “Right now. I’m concerned about the amount that’s being offered in [Obama’s] budget.”

Sen. Mary Landrieu (D-La.) is uncomfortable with the spending levels. Sen. Evan Bayh (D-Ind.) is uncomfortable with the tax increases on the wealthy, and wants to see the federal government cut back during an economic crisis. Sen. Mark Begich (D-Alaska) wants to see investments cut off after a year. Sen. Jon Tester (D-Mont.), apparently talking about spending increases, asked, “At what point of time do you say enough is enough?”

We’ve reached the point at which Evan Bayh has an op-ed in the Wall Street Journal, complaining about earmarks and taxes, which Senate Minority Leader Mitch McConnell (R-Ky.) liked so much, he entered it into the congressional record this morning.

I can appreciate Democratic lawmakers wanting to have an influence over the budget process. Obviously, they’re not rubber-stamps for the White House, nor should they be.

But based on the concerns they’ve raised, it sounds like far too many “centrist” Democrats aren’t able to shake the mindset that’s dominated politics for the last 30 years. They’re uncomfortable about going too fast. They’re burdened by fears and timidity. Ambition is bad. Change is fine, just so long as it’s slow.

They’re going to need a push.

Steve Benen

Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.