Now that the major parties have all but decided who their nominees will be – with the official nominations coming this July – leaders in both parties are eager to unify their respective bases and discourage voters from considering independent and third party alternatives.

Yet this task will prove especially difficult this election season, as the presumptive nominees, Hillary Clinton and Donald Trump, both have unfavorability ratings above 50 percent.

Many members of both parties are dissatisfied with their nominees: 4 in 10 Republicans have yet to commit to voting for Trump in November, and 25 percent of Republican voters who did not vote for Trump in the primaries say they will "seriously consider" a third party candidate. On the Democratic side, 61 percent of Sanders supporters have a negative view of Clinton, and 27 percent claim they will not vote for her in the general election.

It is this level of dissatisfaction that explains the widespread enthusiasm for an alternative to Trump and Clinton. A recent survey reveals that 55 percent of voters favor having an independent on the ticket this fall.

Yet the leaders of both parties are leveraging their members' distaste for their opponent to dissuade voters from supporting independent and third party candidates, using fear to unify members around their future nominees.

RNC Chairman Reince Priebus, for example, told "Fox News Sunday" that the attempt to recruit and run a conservative alternative in November would bolster Clinton's chances of winning and result in a "suicide mission for our country."

"What it means is that you're throwing down not just eight years of the White House, but potentially 100 years on the Supreme Court and wrecking this country for many generations," he told host Chris Wallace. "I think they should consider the ramifications of what's going to happen on the Supreme Court, get assurances from Donald Trump that they're satisfied with that would show that he's committed to those conservative justices ... and I think that's the better way to go as opposed to this third party route."

Debbie Wasserman Schultz, the chair of the DNC, has made dire claims about the effects of a Trump presidency in order to forge party unity. A recent fundraising package signed by Wasserman Schultz lists five reasons to "STOP DONALD TRUMP" and claims that "The Republicans, led by Donald Trump, want to...destroy all the progress we've made over the past eight years."

Wasserman Schultz is leading the effort to secure the loyalty of Sanders and his supporters through concessions related to the make-up of several convention committees, but the Sanders campaign, which has been especially critical of Wasserman Schultz's stewardship of the DNC, appears to have little interest in kowtowing to calls to "dial down his rhetoric" against the party establishment and Clinton.

Indeed, in a move indicative of his history of political independence, Sanders recently endorsed Wasserman Schultz's Democratic primary opponent in Florida's 23rd district, Tim Canova.

Aware that Sanders has agreed to support the party's eventual nominee, Clinton too has argued that the party will unite behind her by pointing to the prospect of a Trump presidency. “I have every confidence that we’re going to be unified,” Clinton told CNN. “I think what brings us together is Donald Trump.”

Other Democrats are making similar 'support-us-to-defeat-the-enemy' arguments to cultivate party unity.

"I am convinced, as Bernie has said repeatedly, he is going to be on the team to defeat Donald Trump," Illinois Sen. Dick Durbin has said. And Connecticut Sen. Chris Murphy has stated that "Donald Trump is going to be a fantastic unifying force within the Democratic Party."

On the Republican side, the arguments are of the same 'lesser-of-two-evils' variety.

Senate Majority Leader Mitch McConnell, for example, has asserted that since “Hillary Clinton will be four more years of Barack Obama...that’s going to, in the end, be enough to unify Republicans across the country."

Even House Speaker Paul Ryan, who has stated he is not yet ready to officially endorse Trump, has long argued that any of the GOP candidates, Trump included, would make for a better president than Clinton.

After their meeting in mid-May, Ryan and Trump issued a joint statement claiming that their conversation signaled a "a very positive step toward unification," citing their common commitment to avoid "another four years of the Obama White House, which is what Hillary Clinton represents."

While in recent days there has been a series of upbeat events with Trump and a number of conservative power bases that are symbolic of growing GOP unity, some Republicans are insistent that they will never support Trump. The #NeverTrump movement is continuing its search for an alternative candidate, and the Libertarian Party is increasingly gaining traction among a sizable portion of the electorate.

On the Democratic side, though most Sanders supporters will pull the lever for Clinton, and even a small percentage claim to prefer Trump, others are considering third party and independent candidates. Significantly, 75 percent of Sanders supporters said in a West Virginia exit poll that they would not support Clinton in the general election,

Unfazed by DNC efforts to accommodate Sanders supporters at the national convention in Philadelphia, some members of the #NeverHillary movement have professed their interest in voting for the Green Party nominee. One online pledge calling for voters to either write in Sanders in November or vote Green has more than 100,000 signatures

On ABC's This Week, Sen. Sanders recently sympathized with the large number of Americans who are frustrated with the two main choices. "If you look at the favorability ratings of Donald Trump and Hillary Clinton, both of them have very, very high unfavorables," he said, adding, “I don't want to see the American people voting for the lesser of two evils."

With the general election several months away, disappointed voters will soon have to make up their minds: will they take the advice of the leaders of the major parties, and support their nominees against the enemy on the other side, or will they vote their conscience and back the candidates whom they are for?