Mercedes duo Nico Rosberg and Lewis Hamilton have dominated the early stages of the season, and Horner believes that Red Bull's title hopes could hinge on how successful they are on their visit to Spain.
"We've got to if we're going to make a championship of it. We've got to take the fight to them," Horner told Sky Sports News.
"We're going to give it everything. I believe we can take the fight to them, we just can't concede too much more ground."
Rosberg claimed victory on the opening weekend in Australia, while Hamilton has won the past three races.