DENVER — The bitter partisan gridlock facing Congress really isn’t a problem in Tennessee. Or California. Or North Dakota.

The same voters who re-elected the Republican House and Democratic Senate also swept in one-party rule in a whopping 45 state legislatures, up from 41 in 2010, according to the National Conference of State Legislatures.

What’s more, 25 states have veto-proof majorities, meaning that one party has enough legislators to override a gubernatorial veto if they choose. That may not be a problem, given that in 22 of those states, the same party that controls the legislature is also in charge of the governor’s office.

Just four states — Iowa, Kentucky, New Hampshire and Virginia — kept divided legislatures, the lowest benchmark since 1944. Nebraska has a unicameral, nonpartisan legislature.

The phenomenon represents an incredible show of unity from a state perspective and a worrisome sign of Balkanization at the national level. Illinois and Indiana may share a border, but not much else. Illinois has a veto-proof Democratic majority, while Indiana has the same thing on the Republican side.

“There’s polarization at the state level that we haven’t seen in recent history,” said Karl Kurtz, director of the Trust for Representative Democracy in the Denver office of the National Conference of State Legislatures.

“I think it’s a reflection of the growing polarization of our society. The fact that you had only 10 battleground states in this presidential election — there’s this standing decision that voters of most states have made to vote for one party or another, and that’s playing out at the legislative level as well,” he said.

The result could be a nation that, at least for the next few years, tests the limits of federalism and the states’ traditional role as laboratories of democracy. Americans may find marriage laws, tax rates and drug enforcement changing dramatically as they cross state lines.

Or the monolithic one-party states could find that absolute power isn’t as easy to handle as it looks. States with longtime one-party dominance often find the majority splits into factions, particularly on emotion-laden social issues, which can erode party unity.

“A lot of legislators will tell you it’s harder to lead a big majority than a small majority,” Mr. Kurtz said. “Not every vote is needed on every bill, so some legislators may break off on certain issues, and then party discipline breaks down, or you get factions.”

The party breakdown shows Republicans holding majorities in both legislative houses in 26 states, the same number they had prior to elections this year. Democrats increased their hold from 15 to 19 state legislatures in which they control both chambers after winning back some seats that they lost in the 2010 Republican wave.

In three of the four legislatures that remain split, the trend shows that they may line up soon for one party or the other. Moving Democratic are Iowa and New Hampshire, where Democrats made gains this year. Meanwhile, Republicans are poised for a legislative takeover in Kentucky as they continue to chip away at the Democratic lead in the state House. In Virginia, Republicans hold a two-thirds majority in the House of Delegates, but the Senate is split evenly at 20-20.

Another danger is that the majority party may go on a binge and push through legislation that tests the tolerance of the electorate. Analysts will be watching California, where Democrats gained two-thirds majorities in both houses, meaning they can override vetoes and approve tax increases without voter approval.

Given that voters just passed a tax increase with Proposition 30, this might not be the best time for Democrats to seek more revenue. If they are unable to restrain themselves after years of budget cuts, California Gov. Jerry Brown, a Democrat himself, may be playing the role of Scrooge.

Moderate governors also could tangle with legislators. In Colorado, Gov. John Hickenlooper, a Democrat who takes a pro-growth stance on economic and energy issues, faces a more liberal one-party legislature after Democrats retook control of the House.

“For Hickenlooper, this is a bad thing,” said Denver pollster Floyd Ciruli. “As a moderate governor, he’s much better off having a check and balance that acts as a moderating influence on the legislature.”

Not every state requires a supermajority to override a gubernatorial veto. In some states, a veto may be overcome with a simple majority of the legislature, while in other states, an override may require two-thirds, three-fourths or three-fifths votes of both chambers.

Other executives on the hot seat are the three governors facing veto-proof majorities from another party. Both houses of the Arkansas General Assembly flipped from Democratic to Republican control on Election Day. Democrat Mike Beebe is the governor of the state, which requires only a simple majority for an override.

Another Democrat, Missouri Gov. Jay Nixon, will continue to deal with veto-proof Republican legislative majorities. Rhode Island Gov. Lincoln Chafee, an independent, also will have to wield a veto that one party has the votes to override, although the former Republican senator supported President Obama’s re-election and sides with Democrats on most issues.

One-party rule was common in the 1960s and 1970s, but for different reasons. Prior to the 1980s, Democrats dominated state legislatures in the South and regularly held veto-proof majorities, while Republicans rarely held such control outside states such as Utah.

As of the latest tally, Republicans account for 16 of the 25 states with veto-proof majorities, while Democrats represent nine. Many of those Republican states are in the South, which has all but completed its full political-party makeover.

“The difference today is that most of the states with two-thirds majorities are Republican,” Mr. Kurtz said. “And that’s never happened before.”

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