File: Iowa Republican Senatorial candidate Joni Ernst speaks during a campaign rally as former Alaska governor Sarah Palin, left, looks on, Sunday, April 27, 2014, in West Des Moines, Iowa. (AP Photo/Charlie Neibergall)

Iowa's primary on Tuesday could lead to something we've seen only once in the last 50 years: the parties picking their candidates by convention.

If no candidate gets 35 percent of the vote, voters will effectively cede their nominating power to the state party convention later this month. So while state Sen. Joni Ernst (R) is the clear favorite for the Republican U.S. Senate nomination heading into Tuesday, if she falls short of that threshold, all bets are off.

The rarely-invoked rule highlights the odd ways that several states nominate their candidates. Below, we rank the most bizarre of them all:

1. IOWA -- The "35 or everyone's alive" primary

Some states attempt to nominate or endorse candidates through state party conventions before the primary. In Iowa, they use the convention as a backup plan.

And the 35 percent threshold isn't just an issue in the Senate race; it's also quite possible it will settle the GOP primary in the all-important 3rd district race and possibly even the Democratic primary in the northeastern 1st district, which could be in-play come November.

Conventions have historically meant that underfunded and more ideological candidates have a better shot, so whether or not things proceed in that direction matters.

Notable historical example: This actually hasn't happened since 2002. That year, the state GOP nominated one Rep. Steve King (R-Iowa), who is now one of the leading conservative crusaders in Congress. Prior to that, it hadn't happened since 1964!

2. UTAH -- The all-important convention

In no state is the state party convention more important. That's because candidates will, more often than not, win the nomination outright before a primary is ever held.

Both state party conventions feature multi-ballot processes in which the field is narrowed until one candidate gets 60 percent of the vote. If there are just two candidates left and nobody gets to 60 percent, it goes to a primary.

Notable historical example: Sen. Robert Bennett (R-Utah) didn't even make the final two in 2010, leaving now-Sen. Mike Lee (R-Utah) and former congressional candidate Tim Bridgewater to duke it out in the primary. Bridgewater, notably, just missed attaining 60 percent of the vote in the final tally, which would mean he would be senator today rather than Lee.

Sen. Mike Lee (R-Utah) addresses a crowd during a rally at the Western Republican Leadership Conference Friday, April 25, 2014, in Sandy, Utah. (AP Photo/Rick Bowmer)

3. LOUISIANA -- The "jungle" primary

If you're waiting for the Louisiana primary, keep waiting. This is the only state that holds its primary -- a nonpartisan one -- on Election Day. If no candidate gets to 50 percent plus one, the top two go to a runoff in December -- a month after election season ended for everybody else.

The system was instituted (naturally) by former governor Edwin Edwards (D), who didn't really like having to run a primary campaign and a primary runoff campaign against Democrats before heading to the general election.

Notable historical examples: Sen. Mary Landrieu (D-La.) who seems likely to face a December runoff this year, has needed them in two of here three campaigns for Senate. The best runoff, though, is probably the 1991 governors race, in which Republicans split up 64 percent of the vote on Election Day, but Edwards won the runoff over former KKK grand wizard David Duke (R). No wonder Edwards picked that system.

4. CALIFORNIA/WASHINGTON -- The "top two" primary

Both of these states effectively have Louisiana's system, with two differences: 1) the open primary is held well before Election Day, and 2) the top two candidates proceed to the November general election, regardless of whether one of them got more than 50 percent the first time.

California is also holding its primary on Tuesday, so expected plenty of talk about this system Tuesday.

Notable historical example: California Democrats fell victim to this system in its first year. In Rep. Gary Miller's (R-Calif.) district, Democrats split up the vote so much that two Republicans advanced to the general election -- in a 57 percent Obama district. 

Republican gubernatorial candidate, Virginia Attorney General Ken Cuccinelli, right, and running mates Lt. Gov candidate, E.W. Jackson, center, and Attorney General candidate State Sen. Mark Obenshain, in October 2013. (AP Photo/Steve Helber)

5. VIRGINIA -- Parties' choice

Virginia law allow for nominating candidates via either a party convention or via a primary -- whichever either party wants to do in any given race. Not surprisingly, the parties generally like to nominate via convention, because it means they have more control over the process.

The upshot is that conventions tend to pick more ideologically pure candidates, while moderates stand better chances in primaries, where other things come into play, including fundraising and campaigning.

(This also means that, as candidates are trying to decide whether to run in the first place, one of the biggest determinants will generally be which method is chosen.)

Notable historical examples: In 2008, moderate Rep. Tom Davis (R-Va.) opted not to run for Senate after the state party chose to nominate by convention. And last year, the state GOP's decision to use a convention effectively pushed Lt. Gov. Bill Bolling out of the GOP race against Attorney General Ken Cuccinelli, the more conservative candidate.

It also meant the GOP nominated E.W. Jackson, a divisive conservative candidate, for lieutenant governor. Just two years earlier, Jackson took less than 5 percent (NOT a typo) in the state's GOP Senate primary against former senator George Allen (R-Va.).