Dear voter: Want to know why Democrats and Republicans in Congress find it so hard to work together to solve tough problems like the debt ceiling, health care and Social Security?

Look in the mirror.

Americans gripe about cowardly, self-serving politicians, and Congress doubtlessly has its feckless moments and members. But voters are quick to overlook their own role in legislative impasses that keep the nation from resolving big, obvious, festering problems such as immigration, the long-term stability of Medicare, and now, the debt ceiling.

Here's the truth: The overwhelming majority of senators and House members do what their constituents want them to do. Or, more to the point, they respond to people in their districts who bother to vote. Nothing is dearer to politicians than re-election, and most have a keen sense of when they are straying into dangerous waters.

For a growing number of senators and representatives, the only risk is in their party's primary, not in the general election. Most voters, and many news outlets, ignore primaries. That gives control to a relative handful of motivated, hard-core liberals (in Democratic contests) and full-bore conservatives (in GOP primaries).

In politically balanced districts, a hard-right or hard-left nominee may have trouble in the general election, when many independent and centrist voters turn out. But many House districts today aren't balanced, thanks largely to legislative gerrymandering and Americans' inclination to live and work near people who share their views and values.

The result is districts so solidly conservative that no GOP nominee can possibly lose, or so firmly liberal that any Democratic nominee is certain to win. In these districts, the primary is the whole ball game.

Republican lawmakers are under constant pressure to drift to the right, to make sure no fire-breathing conservative outflanks them in a light-turnout primary dominated by ideologues. The same goes for Democrats on the left.

So who turns up on Capitol Hill for freshman orientation? Democrats and Republicans who can barely comprehend each other's political viewpoints, let alone embrace them enough to pursue a possible compromise on big issues.

But what if a Republican and Democrat do decide to meet halfway in hopes of finding, say, a path to shore up Social Security for decades to come. What can they expect?

In some states and districts, they can expect to be drummed out of their party for the crime of engaging with "the enemy." That's what happened last year to Bob Bennett of Utah, a mainstream conservative Republican senator. A relatively small number of conservative activists, led by tea partyers, bounced him from the ticket at a GOP convention. They taunted Bennett with chants of "TARP, TARP." He had voted for the bipartisan bank bailout legislation pushed by Republican President George W. Bush. The Senate's GOP leaders also voted for the bill. But it was an unacceptable compromise in the eyes of Utah Republicans picking their Senate nominee.

In Alaska, GOP primary voters also kicked Sen. Lisa Murkowski off their ballot. She barely saved her seat with a scrappy write-in candidacy. Murkowski supported the bank bailout and, admittedly, is more moderate than the average congressional Republican. But her improbable write-in victory proved she is popular with Alaskans in general, even if her own party rejected her in the primary.

Tea party leaders spell out a warning in their periodic Washington rallies.

"The message is that we're watching, and we want you to vote based on our core values," Mark Meckler, a co-founder of the Tea Party Patriots, said at one such event.

When Democratic leaders were struggling earlier this year to strike a budget deal and avert a government shutdown, Phil Kerpen of the conservative group Americans for Prosperity said sharply, "No Republican better help them." The crowd cheered loudly.

Such threats are mainly aimed at Republicans for now, largely because of the tea party's rapid rise. But Democratic lawmakers also know liberal discontent might undo them if they stray too far to the center.

"It's astounding how often some Democratic leaders sacrifice principles when critical issues are at stake," said a writer for the liberal AmericaBlog. The column rebuked Sen. Dick Durbin, D-Ill., for working with the bipartisan "Gang of Six" on a debt-reduction plan.

A McClatchy-Marist poll this year found that 71 percent of registered voters want political leaders in Washington to compromise to get things done. If those voters skip key primaries, however, they may have little say in the matter. Political enthusiasts, whether they wear peace signs or "Don't Tread On Me" T-shirts, will determine who gets elected in many districts before a wide swath of Americans even notice it's an election year.

Except for a recently appointed senator from Nevada, every member of Congress got there the same way: American voters elected them.

People may bristle at the notion that we get the government we deserve. But there's no denying we get the government we elect.