Singular or plural verb?

[Jazst]

"The monitoring and assessment of public policies constitute/constitutes one of the cornerstones of democracy."

Should I use the singular or the plural form? This is translated from a proofread text in Slovene (a singular verb is used in the original text, though a plural verb would generally be used).

Any explanations and suggestions would be greatly appreciated.

[Alan]

Both are possible, depending on viewpoint, but where in doubt, a plural verb is invariably advisable with a compound subject.