Iowa Democratic Sen. Tom Harkin's retirement strengthens the Republican Party's chances of picking up a seat that has been in Democratic hands for nearly 30  years. It also presents the party with a crucial test: Will it nominate the most electable Republican or a conservative with a strong base but limited potential in November?

As things stand, both are real possibilities.

Rep. Steve King (Cliff Owen/AP)

The big question for Republicans is whether Rep. Steve King (R) will run. The outspoken, staunchly conservative tea-party favorite has at least been thinking about it (based on his public comments). While King has a loyal base, he would likely have a hard time extending his reach beyond it in a general election, particularly in the eastern part of the state. Even Gov. Terry Branstad (R) has acknowledged as much.

From an electability standpoint, the GOP would be better served to take a look at Rep. Tom Latham, a more moderate Republican with the potential for broader appeal. Another name that has been tossed around in GOP circles is Lt. Gov. Kim Reynolds.

Latham spokesman James Carstensen wouldn't tip the congressman's hand on Saturday, saying: "Congressman Latham respects Senator Harkin's decision. He looks forward to continuing to working with him and the rest of the Iowa delegation for the best interests of the people of Iowa over the next two years."

Since King was already toying with the idea of running, even before Harkin's retirement, the National Republican Senatorial Committee was already faced with a potential test-in-waiting in Iowa. Burned by flawed nominees in 2012 -- and in this piece we get into the details of why King would be flawed -- would it opt to intervene and try to keep King away? Now, with an open seat that is sure to attract more interest from all potential candidates, the committee may be forced to confront that decision sooner rather than later.

The NRSC didn't comment on its specific plans in a Saturday statement expressing confidence about the race. "Today's announcement by Senator Harkin immediately vaults Iowa into the top tier of competitive Senate races next year," said NRSC Executive Director Rob Collins.

Harkin's political standing wasn't great in the state, but it wasn't disastrous, either. A recent survey from Democratic-leaning pollster Public Policy Polling showed that voters were evenly split over the job he was doing. Harkin led a generic Republican opponent by eight points in the survey.

In other words, with the natural advantages of incumbency, Harkin would have had a better than fair chance of winning a sixth term.

But now, it will be more difficult for Democrats. Look for party strategists to try to woo Rep. Bruce Braley, a strong fundraiser who cruised to reelection in 2012. Braley would be a formidable Democratic contender who is already familiar with the rigors of a federal campaign.

Another big name to watch on the Democratic side is Agriculture Secretary Tom Vilsack, the state's former governor. Vilsack's wife, former Iowa first lady Christie Vilsack, who lost to King in 2012, is also a possible candidate.

Harkin made his announcement on the same day as the annual meeting of the Iowa Democratic Party, so there will  be lots of discussion there about the seat. The Senate Democrats' campaign arm, meanwhile, lauded the timing of Harkin's announcement.

"I appreciate that Senator Harkin has made this decision so early in the cycle, giving us ample time to recruit a strong Democratic candidate for this seat," said Democratic Senatorial Campaign Committee Chairman Michael Bennet.

President Obama carried Iowa twice, most recently in 2012 by nearly six points, although it's still lately regarded as a swing state. With the right candidate, the state's Democrats have a chance of holding the seat.

That right candidate question is equally important for Republicans, who are already looking at a very favorable 2014 map ripe with pickup opportunities.

Iowa just got more interesting for the GOP. Whether they will take advantage or let a new opportunity slip by the wayside remains to be seen.

-- Ed O'Keefe contributed.