Rush Limbaugh tried something similar in 2008 (encouraging Republicans to vote for Hillary in Obama-leaning states), and it just made him look like a bigger d-bag than he already was.
I think it's dangerous and unethical to vote for a candidate other than because you genuinely want them to win. Dangerous because it can backfire, and unethical because it subverts trust in the democratic process and its outcome--however flawed they may already be.
I know that some states allow voters to vote in primary elections of parties with which they're not affiliated. I think this is just fine if those voters pick candidates who they genuinely want to see succeed. For example, had I been eligible to vote in Republican primaries in 2008, I probably would have voted for John McCain. In the general election, I preferred Obama, but I also genuinely felt that McCain was the most competent, intelligent, and deserving Republican candidate, and I wanted to see him succeed and make the general election campaign competitive and thoughtful.
If nothing else, Democratic voters should understand that they're doing their party no favors by setting them up with a weak opponent. Politicians without credible, thoughtful opposition tend to act like ordinary politicians... which is to say that they become corrupt, authoritarian, lazy, and devoid of ideas. Many American leaders have been most productive when they've faced smart opponents... and won. I'm thinking about Bill Clinton vs. Newt Gingrich and Ronald Reagan vs. Mikhail Gorbachev, in particular.