While Texas Democrats and Republicans hold their state conventions this weekend, two other parties on the ballot will also be gathering — for their equivalent of last month's primary.

The Libertarian Party and the Green Party, unlike the two major parties, have not yet voted on their nominees for statewide office. In the handful of positions for which they have more than one candidate running, delegates at this weekend’s conventions will decide whose names will appear before voters on the November ballot.

The Libertarian Party convention will be held in a hotel near the Dallas/Fort Worth International Airport. The biggest draw of the three-day event will be former New Mexico Gov. Gary Johnson, who accepted the party’s nomination for president at the party's national convention in May.

“When you have a candidate as experienced as Gary Johnson, that is an impressive résumé to carry, and there may be some coattails” in the general election, said Pat Dixon, chairman of the Libertarian Party of Texas.

The Green Party's convention, just outside of San Antonio, will feature three presidential candidates who have campaigned nationally. Jill Stein, a Massachusetts physician whose campaign says she secured the party’s nomination this week, will attend the convention, while two other candidates will be screened in through a video call.

Neither party has an elected official at the state level, and the chances of that changing this November are low. But fielding candidates in races — even where there’s little chance of winning — is important for the party to get out its message, Dixon said.

“You’ve got to be on the ballot and be in the game to grow,” he said.

Both the Libertarian and Green parties have candidates for statewide offices. They also have candidates in some races where one of the two major parties is sitting out. Past results suggest these races will draw the highest vote totals for the Libertarian Party or the Green Party. The parties also perform better in county-level races.

Perhaps the biggest influence of the smaller parties on election outcomes happens in November with the so-called spoiler effect — when a close race between a Democrat and a Republican is influenced by a third-party candidate. In 2010, for example, state Rep. Donna Howard, D-Austin, defeated her Republican opponent by four votes. Ben Easton, the Libertarian candidate, got 2.9 percent of the vote — much more than would have been needed to tip the scale in the Republican candidate’s favor.

Dixon said he disagrees with the assumption that those voters would have voted Republican if the Libertarian candidate hadn’t run. He also said the answer to the spoiler effect is approval voting, in which voters check off every candidate that appeals to them instead of choosing one. He said the method is gaining traction among Libertarians nationwide.

The Libertarian Party of Texas will be putting approval voting up for a vote before its 210 delegates at this week’s convention. If accepted, the Libertarian nominee for U.S. Senate will be chosen using the method (six candidates are running).

Approval voting is "the way most people make decisions,” Dixon said. “That’s the way elections ought to be, too. It shouldn’t be penalizing people for making a choice. It should be empowering people to say these are the acceptable choices and these are not.”

The Green Party will have 30 to 40 delegates at its convention, said David Wager, the Texas co-chairman. He said his party seeks to offer an alternative to liberals disenchanted by the Democratic Party.

“I have seen a lot of progressive Democrats in Texas leaving the party because they have tried for the past 20-odd years to implement changes in the Texas Democratic Party, to no avail,” Wager said. “The Green Party of Texas has captured a lot of those people.”

The two parties will get their candidates on the general election ballot without a petition because they had candidates in the 2010 elections that won more than 5 percent of the vote in a statewide race. The Libertarian Party had three such candidates. The Green Party crossed the threshold in the comptroller’s race, in which no Democrat was running.