Love him or hate him, Sen. Pat Roberts (R-Kansas) is honest with his constituents about what his reelection would mean. He is a Republican. If he won, he would caucus with Republicans. He would vote like a generic Republican. He would in a marginal way promote the advancement of the overall Republican agenda. If Kansans like that agenda, they should vote for him. If they don't like it, they shouldn't. It's very straightforward.
By contrast, Roberts' opponent, independent Greg Orman, is like Schrödinger's candidate. Kansans have no idea what his election would mean. He has promised to caucus with whichever party wins a Senate majority. He has his own issue platform, sure, but that hardly matters. The most important fact about the Senate isn't what individual senators believe, but what the body's overall party makeup would be.
A GOP-majority Senate would have substantially different consequences for the country than a Democratic-majority one. Executive and judicial nominations would be delayed, and some blocked outright. Legislation that President Obama opposes but not passionately enough to use his veto power — like, say, a bill approving the Keystone XL pipeline — might make it through. What Orman's candidacy tells Kansans, essentially, is, "You will have no say in whether Obama is able to name a solid liberal to a Supreme Court vacancy. You will have no say in whether Keystone XL passes. By electing me, you will forfeit any control over the Senate to other states."
The King possibility
There are two possible defenses to make of Orman here. The most charitable reading is that Orman has no intention of ever caucusing with Republicans. The analogy here is Sen. Angus King (I-Maine), who throughout his 2012 campaign insisted that he wasn't sure what party he'd caucus with, even though he was obviously going to caucus with the Democrats. A New York Times post announcing that King, post-election, had decided to do just that begins, "Surprising no one…"
Orman, like King, is running on what's basically a Democratic platform. The issues section of his website states that he's pro-choice, backs a Constitutional amendment to overturn Citizens' United, wants to close the gun show loophole to background checks, opposes the Supreme Court's ruling in Hobby Lobby, and wants a path to citizenship for undocumented immigrants — although, hilariously, he has the chutzpah to run ads implying he'd be harsher to undocumented immigrants than Roberts:
But there are strong reasons to think this is not a King scenario, not least of which is that Orman has actually promised to caucus with the party that wins a majority — a promise King never made. If control on the Senate hinges on him, it's reasonable to think he'd caucus with Democrats; FiveThirtyEight's model assigns a 75-percent probably to that, while the Upshot's just assumes he'd caucus with Democrats in that case. But he is definitely caucusing with Republicans, should they get to 51 seats without him. That was never true of King.
The Chafee possibility
The other possible defense of Orman is that while his party affiliation is up in the air, he'll vote like a Democrat no matter which party he chooses. The best analogy here is current Rhode Island Gov. Lincoln Chafee, now a Democrat, who served in the Senate for seven years as a Republican, voting more or less like a mainline Democrat all the while.
But Orman is not going to be like Chafee. Republicans didn't like that Chafee voted the way he did, but they put up with it for understandable reasons. He represented Rhode Island, which probably wouldn't stand for a more conservative Senator; as it was, Sheldon Whitehouse beat Chafee in 2006 by running to his left. Chafee was also the son of a late, beloved Republican Senator. Perhaps most importantly, the Republican party of 1999 to 2007 was noticeably less conservative, and more tolerant of ideological variation, than the Republican party of 2014 is.
By contrast, if Orman decides to caucus with the Republican majority, then-Majority Leader Mitch McConnell will probably present him with two options. Option one, he gets valuable committee assignments and gets to offer his amendments and gets to see his bills considered on the floor and gets support from other Republicans on issues of importance to Kansas and gets support from the party when he runs for reelection and, in general, gets to represent Kansas effectively, just like he promised he would. He also votes like a Republican on major bills, with maybe a few freebie defections here and there.
Option two, he gets frozen out of the good committees and watches Republicans bottle up his bills and ignore his amendments and generally make Kansas rue the day they elected Orman to represent them. Hell, maybe he even gets expelled from the caucus and has to run for reelection with Kansans knowing he couldn't even keep his promise to caucus with the majority.
Orman will pick option one. Anyone would pick option one. And he'll vote more or less like Pat Roberts from then on.
The bottom line
The way that representative democracy works, or should work, is that candidates offer competing policy platforms, voters choose which of those platforms they prefer, and then whichever platform most elected candidates support gets enacted. There are a whole lot of things preventing America from being that kind of ideal representative democracy, but our increasingly strong party system at least means the "candidates present different visions which voters can evaluate" part is true.
Orman is living proof that candidates, if they are cynical enough, can deny voters even that much. He has set up his campaign in such a way that Kansans will either get a mainline Democrat or a mainline Republican if he wins, depending on which results in him becoming more personally powerful in the Senate, and there's no way for them to influence which it'll be. For Democrats that's a better value proposition than Roberts offers, and it's sensible enough for Kansas Democrats to support Orman on those grounds. But Kansas's voters shouldn't be happy about it.