Campaign slogans usually take the form of expressing a key campaign issue or draw attention to a particularly contentious issue which is dividing the constituents.  Campaign slogans also "polk" fun at candidates.  Play on the name of the candidate, such as a rhyme or illiteration, can also make for a catchy slogan.  Sometimes campaign slogans come into being spontaneously as a phrase from a speech sticks and becomes a slogan. 

Some examples from past campaigns in the US and Britain:

• Bargain and Corruption (Andrew Jackson 1828)
• Tippercanoe and Tyler Too (William Harrison 1840)
• Who's Polk? (James Polk 1844)
• 54-40 or Fight (James Polk 1844)
• We Polked You in 1844, We Shall Pierce You In 1852 (Democrats 1852)
• Free Soil, Free Men, Free Speech, Fremont (John Fremont 1856)
• Anything To Beat Grant (Horace Greeley 1872)
• Hayes, Hard Money And Hard Times (Democrats 1876)
• Rum, Romanism, Rebellion (Republicans 1884)
• Full Dinner Pail (Republicans 1900)
• You Can't Beat Somebody With Nobody (Theodore Roosevelt 1904)
• He Kept Us Out Of War (Woodrow Wilson 1916)
• Back to Normalcy (Warren Harding 1920)
• Keep Cool With Coolidge (Calvin Coolidge 1924)
• Hoover And Happiness Or Smith And Soup Houses (Herbert Hoover 1928)
• In Hoover We Trusted, Now We Are Busted (Franklin Roosevelt 1932)
• Land Landin With A Landslide (Alfred Landon 1936)
• We Want Wilkie (Wendle Wilkie 1940)
• Time For A Change (Thomas Dewey 1944)
• Don't Let Them Take It Away (Harry S. Truman 1948)
• Give 'Em Hell Harry (spontaneous slogan in support of Harry Truman that came out of a speech Truman gave in 1948)
• I LIke Ike (Dwight Eisenhower 1952 - Irving Berlin used a variation of this slogan "They like Ike."  earlier in a song written in 1950 for the musical Call Me Madam)
• Let's Get America Moving Again (John Kennedy 1960)
• Keep The Peace Without Surrender (Richard Nixon 1960)
• All The Way With LBJ (Lyndon Johnson 1964)
• In Your Heart You Know He's Right (Barry Goldwater 1964)
• Nixon's The One (Richard Nixon 1968)
• Send Them A Message (George Wallace 1972)
• He's Making Us Proud Again (Gerald Ford 1976)
• I'm Jimmy Carter And I'm Running For President (Jimmy Carter 1976)
• The Time Is Now (Ronald Reagan 1980)

And from Britain:

• Yesterday The Trenches, Today The Unemployed (Ramsey MacDonald 1923)
• Safety First (Stanley Baldwin 1929)
• Send Him Back To Finish The Job (Winston Churchill 1945)
• Your Future Is In Your Hands (Liberals 1950)
• Make Britain Great Again (Liberals 1950)
• Life's Better With The Conservatives - Don't Let Labour Ruin It (Conservative Party 1959)
• Let's Go With Labour, And We'll Get Things Done (Labour Party 1964)
• You Know Labour Government Works (Labour Party 1966)
• Yesterday's Men (Labour Party 1970)
• Britain Will Win With Labour (Labour Party 1974)
• Labour Is The Answer (Labour Party 1979)